Ans: D
Note that $BC$ is the line of intersection of planes $ABC$ and $DBC$. Since $AE \perp BC$ and $DE \perp BC$, then the angle between the plane $ABC$ and the horizontal ground is $\angle AED$.
Note that $BC$ is the line of intersection of planes $ABC$ and $DBC$. Since $AE \perp BC$ and $DE \perp BC$, then the angle between the plane $ABC$ and the horizontal ground is $\angle AED$.
